1. Field of the Invention
This invention relates to a GPS receiver, and in particular to a solid state GPS receiver that provides continuous carrier phase tracking. The GPS receiver of the invention maybe used, by way of example, to provide real time control or guidance of heavy machinery, including farm equipment.
2. Description of Related Art
This section begins with a background discussion of the current Global Positioning System (GPS), which is followed by a discussion of the requirements of, and problems associated with, application of GPS to automatic control of heavy machinery and in particular farm equipment, and finally a discussion of specific previously-proposed solutions to some of the problems addressed by the present invention. Those skilled in the art will appreciate that although the present invention addresses a number of problems related to specific applications, the solutions presented may be applicable to a wide variety of different GPS applications, in contexts not specifically discussed herein. The discussion of related art is therefore not intended to be limiting, nor should the related art discussed herein be considered the only prior art having relevance to the present invention.
The Global Positioning System (GPS)
GPS is a satellite constellation originally developed by the U.S. military for real-time 3 dimensional position fixing worldwide. GPS nominally employs 24 satellites in circular 26,000 km orbits. Each satellite carries an atomic frequency standard tied to GPS time as maintained by the U.S. Naval Observatory. The radio signal propagates from the satellite to the user at the speed of light. The ability to determine range to the satellite is therefore governed by one""s ability to measure time.
The normal operation of GPS is to broadcast a timing signal using a known biphase pseudo random noise (PRN) code so that a receiver can lock up to this pre-defined sequence. If a user had a perfect clock, ranging to 3 separate sources (one for each dimension of latitude, longitude, and altitude) would be sufficient to determine one""s position. To allow for low-cost receiver sets that utilize a simple quartz oscillator time base, the design of GPS includes an additional 4th ranging source. Therefore, every position fix includes a solution for a users position and clock bias with respect to GPS time.
GPS broadcasts on two frequencies, L1 and L2. We can define a basic frequency reference of GPS to be f0 (1.023 MHz). L1 is 1540 f0 (1,574.42 MHz), and L2 is 1,200 f0 (1,227.60 MHz). L1 provides a known C/A code that is provided for civil use worldwide at all times. This is a PRN code that is modulated at f0. There is also a P(Y) code provided on both L1 and L2 that is modulated at 10 f0. The default operational mode is that a known P code is broadcast. The U.S. Military may decide from time to time to switch to an alternate unknown (encrypted) Y code. This mode is called xe2x80x9cAnti-Spoofingxe2x80x9d, or AS. Because this code is not known a priori to an enemy in a time of military conflict, an enemy cannot generate a false signal that could be used to mislead an allied receiver about its own true position.
GPS is now used in a wide variety of applications, including transportation, recreation, scientific research, and industry. The specific applications contemplated herein are general and could pertain to a broad range of applications, including surveying, GIS, natural resources, and mapping. Furthermore, GPS is now starting to be used as a guidance and control sensor in aviation, agriculture, construction, and mining.
Precision Control Applications
Precision automatic control of heavy machinery places stringent requirements on the navigation sensors that are used for guidance. Examples of such machinery and applications are aircraft automatic landing, automatic steering of farm tractors, and autonomous operations of mining and construction equipment, including haul trucks. Sometimes the heavy equipment made from steel and hydraulics, such as tractors, haul trucks, dozers, drills, shovels, road graders, etc. are known as xe2x80x9cheavy ironxe2x80x9d.
Farm Tractors
Automatic guidance of farm tractors is starting to provide major new efficiencies to farmers. Some of the first gains are being realized in row crops, such cotton and vegetables. In row crops, it is very important to create straight furrows for planting and subsequent operations. If rows are too close, the cultivating process will shred the farmer""s crop. If the rows are too widely separated, a farmer loses valuable surface area that could be used for planting.
Prior to automatic guidance, a farmer could only carry out operations in good visibility. Fog, dust, or darkness meant an automatic pause or end to operations. Now, with a GPS-based tractor autosteering system, a farmer can operate 22 hours per dayxe2x80x94something he could never do before. An example of such equipment is the AF5001 manufactured by IntegriNautics in Menlo Park, Calif. Such a unit determines the position of a tractor to a centimeter of accuracy, then uses that information to directly control the steering of the tractor to follow straight rows. The lateral accuracy is one inch.
A human operator is still used. After a 12 hour shift, chronic fatigue is effectively eliminated because the operator now spends his time managing the overall quality functioning of the tractor process-especially the appropriate functioning of the implement itself.
Other gains are being recognized for major commodities, such as corn, wheat, and soybeans. By eliminating the row overlap that commonly occurs, farmers will see 10% reductions in fuel, fertilizer, time, and pesticide use.
Cycle Ambiguity Resolution
For precision positioning using the GPS carrier, centimeter-level accuracy is possible only after the integer cycle ambiguities are resolved between the baseline separating each pair of antennas projected into the line of sight to each satellite.
Some methods of cycle ambiguity resolution include Cohen et al. (U.S. Pat. No. 5,572,218), Zimmerman et al. (pending application filed by IntegriNautics), and Rabinowitz et al. (pending application filed by Stanford). Other methods employ xe2x80x9cwide-laningxe2x80x9d using dual frequency GPS carrier phase measurements. Because the two GPS frequencies when differenced have an effective wavelength that is much longer than that of either band by itself, it is possible to use that information to systematically search for the correct set of cycle ambiguities that form the smallest residual solution error. Various algorithms have been developed that sequentially throw out the xe2x80x9cimagexe2x80x9d solutions. The reference (Instantaneous Ambiguity Resolution, by Ron Hatch, Paper Presented Sep. 11, 1990 at Banff, Canada at KIS Symposium 1990) describes one such technique. xe2x80x9cWide-laningxe2x80x9d is a generic approach known in the art.
Precision Requirements
It is envisioned that this invention will be broadly applicable. However, it is also strongly desired that the invention be useable as a guidance sensor of high quality to control heavy machinery to stringent standards of precision, safety, and reliability. Suitability for machine control implies the potential to be integrated into a feedback control loop system controlling a vehicle or machine to 1 inch lateral accuracy that weighs typically in excess of 20,000 lb. In our invention GPS becomes capable of providing the necessary levels of performance, but requires specific augmentation.
The general requirements for precision control of heavy machinery are as follows:
Accuracy (system deviation from truth): Must be 1 cm (one sigma) or better.
Integrity (system ability to provide timely warnings of hazardous readouts): Probability of hazardously misleading information must be better than one failure in a billion landing approaches for civil aviation. Goal for autonomous applications in heavy iron is better than one failure in a million operational per equivalent exposure time of 150 seconds.
Continuity (probability of operating continuously for entire landing approachxe2x80x94150 second exposure timexe2x80x94given proper system function at start of operation): Established at one failure in 10 million approaches. The indirect requirement on the system is that it must have robust, synchronous output (i.e., no skipped outputs during exposure time at an output rate of 10 Hz). Goal for autonomous operations is one failure per 100,000.
Availability (fraction of time the system is able to provide service): System must be available on demand at least 99.99% of the time.
Acquisition Time (time from cold start to normal operation): A few minutes, typically less than two minutes, is all that is tolerable.
Communications: Should be simple, non-synchronous (i.e., relatively insensitive to latency), low bandwidth, be flexible so as to be compliant with any spectrum regulations, and be resistant to r.f. interference.
Environmental: Should be simple with minimum component count. Simplicity offers more demanding environmental performance and better overall reliability.
Power: Base stations must be low power (often use self-contained power source).
Cost: Must be low cost.
Limitations of Previously Proposed Systems and Methods
Westerfield (U.S. Pat. No. 4,622,557) teaches how a GPS front-end coupled to a down converter, sampler, modulator, and transmitter (a xe2x80x9ctransdigitizerxe2x80x9d) can be used to transmit a real-time r.f. replica of an incoming GPS signal as received from a mobile vehicle (such as a sonobuoy). The ground station then processes the real-time information into position fixes.
An analog version (called a GPS xe2x80x9ctranslatorxe2x80x9d) is described in (Well, L. L., xe2x80x9cReal-Time Missile Tracking with GPSxe2x80x9d, Navigation, vol. II: 105-111 (1984)). Here the incoming GPS signal from a missile is downconverted and rebroadcast on a different band for ground processing.
Cohen (U.S. Pat. No. 5,583,513) describes a pseudolite-based reference station that locks onto the GPS satellites signals, modulates alternate, differentiable codes onto the GPS signal, then coherently rebroadcasts the incoming remodulated carrier signal. The mobile unit can pick up this signal and use it as a differential reference station for position fixing.
However, each of these techniques is limited due to the implicit line-of-sight requirement in the link between the mobile unit and the ground station. In fact, none of these techniques is suitable for automatic control of heavy equipment because each is restricted to cases where there is stable (multipath-free) link or line-of-sight between the ground station and the vehicle. Furthermore, they require significant r.f. bandwidth to implement (between 2 and 10 MHz) which is not readily available to civilian users. The chosen band can also be susceptible to interference.
Other variations of prior art include Johnson (U.S. Pat. No. 5,420,592) and Brown (U.S. Pat. No. 5,379,224) who teach systems (such as for radiosondes) that take digital samples of the raw GPS signal, place them into a buffer, and telemeter them to a ground station for post processing into position fixes. The ground station can optionally use a conventional C/A code GPS receiver for differential operation. Variations on this theme running in post processing are given in the following two papers, (i) A. Brown, M. May, B. Tanju, xe2x80x9cBenefits of Software GPS Receivers for Enhanced Signal Processingxe2x80x9d, GPS Solutions, 4(1) Summer, 2000, pages 56-66), in which the authors use the signal processing power of a computer to derive more performance in the presence of jamming or low signal to noise ratio; and (ii) A. Brown, A. Matini, D. Caffery, High Dynamic, Dual Frequency Tracking with a Low Bandwidth Digital Translator, ION GPS-96, September, 1996, in which the raw GPS spectrum is truncated to 2 MHz prior to being broadcast to the ground. The last technique among the solutions of this class is not considered viable for civil applications because it requires classified knowledge of the GPS encryption algorithms to implement and use.
Also, Krasner has several patents (including U.S. Pat. Nos. 5,663,734; 6,133,874; 6,064,336; 5,781,156; 5,945,944; 5,831,574; 6,016,119) that concern positioning in conjunction with cellular telephone operation. He teaches how a central GPS server/reference station sends out specific aiding information to a user receiver. The aiding information includes an approximate satellite doppler estimate for the user, satellite ephemeris information, reference station reference differential corrections, and an r.f. pilot tone over the communication link for calibrating the user crystal oscillator. The user takes samples of incoming GPS C/A code signals and stores them into a buffer. Using the aiding information supplied by the reference station, the user receiver performs a xe2x80x9cfast convolutionxe2x80x9d between a pre-stored representation of the C/A code and the stored samples to derive an estimate of the GPS pseudorange for each satellite. These pseudoranges are combined to calculate a position fix.
However, these solutions are not useful for performing precision, centimeter-level measurements, let alone applying those measurements to automatic control. None of these inventions are able to offer the use of carrier phase for performing precision positioning because they do not track the carrier continuously (if at all), let alone track it in real time. Brown does address solving for the carrier by a technique of iteratively reprocessing the same data until convergence, but only in the sense of a single burst of data and in post processing. It is clear that an iterative, post processing receiver will eventually converge on the correct solution and could therefore be operated quasi real time given enough tolerance on latency, signal dilution, and/or processing power. However, quasi real time control is only a partial solution. What is needed for a generally useful receiver and for real-time control is a robust solution obtained as quickly as possible using minimal processing resources. But perhaps what is most at issue is that none of these approaches can sustain continuous carrier phase measurements from one data burst to the next. This is imperative for preventing cycle slips and especially for real-time control of heavy machinery.
These systems are architected in ways that do not favor real-time machine control. Two of them (Johnson/Brown) calculate the position fix at the reference site. But the solution is needed in real-time at the user site for control purposes. The additional latency that would be introduced by broadcasting distinct solutions for group of many users is undesirable. The other inventions that do allow the user to calculate his own position (Krasner) are oriented around a having a central server that transmits aiding information, including a pilot tone on the communication link frequency. This additional requirement for an r.f. pilot tone may be acceptable for cellular phone applications, but it is costly and even unworkable for machine control because multipath will constantly corrupt the phase of that signal as it skims the earth""s surface. None of these systems are useable for robust, synchronous, carrier-phase positioning required for automatic control of heavy machinery.
One feature of the invention described herein is the combination of a conventional GPS receiver with one that samples and buffers GPS signals for subsequent processing. Krasner (U.S. Pat. Nos. 5,884,214 and 6,104,340) contemplates combining a first conventional GPS receiver with a second sampled and buffered receiver to create a backup in case of weak GPS signals. In his invention he is concerned with losing the first GPS signal if it becomes too weak. Normally, the first conventional receiver runs. If the signal is lost, the second receiver serves as a backup. Because of its design, the second receiver is able to integrate weak signals longer and therefore has a better chance of recovering a good measurement during blockage.
However, these two Krasner patents again do not forsee broader application for general carrier phase tracking and especially machine control required herein-both in their intended application (dealing with signal blockage) and their design (which yields discontinuous output that is unsuitable for tracking carrier phase without cycle slips). Typically, the carrier is corrupted by multipath during blockage conditions and must therefore be flagged as useablexe2x80x94regardless of its processing means, traditional or snapshot. Even in the presence of a solid, unblocked signal, the prior art does not provide for any of the qualities of robust, synchronous, centimeter-level, carrier phase output that are mandatory to conduct machine control.
As mentioned previously, a key requirement for kinematic operation is resolving integer cycle ambiguities. Some methods involving pseudolites and LEO satellites were mentioned previously and will work well, both in general and for machine control. Dual-frequency GPS measurements of carrier phase are another way to resolve integers, as described previously. For machine control, an ideal solution is one which employs many or all of the above techniques simultaneously to maximize performance.
Many existing dual-frequency receivers employ the unknown Y-code signal of GPS and rely on at least some subset of the following assumptions about the GPS signal.
The Y code is the product of the P code and an (unknown) W code
The W code has a bandwidth of approximately 500 kHz
The Y code is modulated identically on L1 and L2
It is possible to square the L2 signal to recover the carrier. Counselman (U.S. Pat. No. 4,667,203) describes a receiver which generates the second harmonic of the carrier. Other receivers that recover the carrier phase of L2 are described by MacDoran (U.S. Pat. No. 4,463,357), Keegan (U.S. Pat. No. 4,972,431), and Meehan et al. (U.S. Pat. No. 6,061,390). While these receivers all perform as expected, one disadvantage they share with respect to machine control applications is that they produce either a half cycle ambiguity in the L2 phase or a somewhat larger (i.e., by more than 10 dB) noise than other receivers.
Lennen (U.S. Pat. No. 5,825,887) describes a system that uses High Gain Antennas (HGAs) to point at each GPS satellite to try to estimate the W code in real time. This estimate is then used by mobile receivers to strip off the unknown Y code. However, this approach requires expensive and complex HGAs to be employed. Farmers, for example, and most other users need a simple and inexpensive approach.
Another class of receiver provides an advantage of both lower noise and full cycle reconstruction at the output. These receivers make more effective use of the GPS signal by exploiting all of the assumptions listed earlierxe2x80x94most notably that the encrypted Y code is the product of the P code and an unknown W code. Some examples of these receivers are Lorenz et al. (U.S. Pat. No. 5,293,170), Litton et al. (U.S. Pat. No. 5,576,715), Woo et al. (xe2x80x9cOptimum Semicodeless Carrier-Phase Tracking of L2xe2x80x9d, Navigation, Vol. 47, No. 2, p. 82), and Lennen (5,610,984).
However, none of the techniques used for deriving dual-frequency observables in these receivers is suitable for the periodic, sampled processing receiver that is the subject of this invention. Obtaining measurements of carrier phase has traditionally required continuous, uninterrupted lock. With a subject invention receiver that can undergo periodic extended gaps in signal processing as it operates, a new means of deriving carrier phase measurements is needed. Traditional tracking loopsxe2x80x94with their attendant start-up transients, acquisition modes, and vulnerability to cycle slips due to discontinuous signal coveragexe2x80x94will not work. A new method of measuring dual-frequency observables that can withstand interruptions in processing without inducing cycle slips is needed to fulfill all the requirements outlined above.
Many different dual-frequency GPS sets are available on the market. There are at least five major manufacturers in the U.S. GPS industry who market such receivers, including some covered by the above listed patents. Each has some means for recovering the encrypted component of the GPS signal in case the military decides to invoke anti spoofing (AS).
A key advantage of dual-frequency receivers for resolving cycle ambiguities is that the process is rapid and reliable if the correct algorithms are used to process the raw measurements. Key disadvantages are that these receivers consume significant power, and they are expensive.
For example, when used as a reference base station on a farmer""s field, several receivers may be needed at once to service an operating area. To minimize operational burden, a compact, inexpensive reference station needs to be able to be set up once by a farmer and run on a minimum of power, such as a battery that is kept charged by a solar cell.
The intrinsic expense of conventional, off-the-shelf dual-frequency receivers means that a farm tractor system component that is actually used less than 1% of the time accounts for almost 50% of the total system cost.
It is therefore an objective of the invention to provide a simple, low-cost, high-performance positioning sensor that can help farmers, precise machine control users, and users in other applications be more efficient.
According to an ideal architecture for the hardware of the invention, the sensor of the invention uses a common clock to drive a standard C/A code GPS chipset, which in turn generates a coherent 40 MHz sampling clock for the L1 and L2 channels. C/A code carrier phase measurements are then taken synchronously with bursts of raw L1 and L2 quadrature samples. To minimize hardware, these samples may, within the scope of the invention, be taken directly at L band, although the illustrated embodiment of the invention includes a downconverting front-end due to practical limitations on the sharpness of current anti-aliasing filters. In either case, two bits preferably encode the raw L1 and L2 samples, an Interface Block capturing the raw data, which, in varying forms of the invention, can buffer, preprocess, or directly pass the data to a computer or other form of digital signal processor (DSP).
The preferred digital signal processing components include an internal software signal generator that synthesizes both the P-code and carrier phase for both L1 and L2. Once the receiver has acquired and tracked the signal, each of these signal components may be fed forward from the C/A code measurements that are being tracked concurrently. The raw, complex samples are computationally mixed to baseband by simply multiplying the generated signal by the incoming. To speed this mixing process, a one-step lookup table may be employed. The mixed signal product is summed with a 2 ms period to wipe off the P-code and rotate the signal to baseband. The resulting L1 and L2 signals are mixed together to wipe off the residual W-code. The remaining signal is a baseband single-difference phase measurement for a given satellite that is essentially xe2x80x9cfrozenxe2x80x9d in the I-Q plane. By integrating for 100 ms or longer, the noise is averaged down to obtain a useable signal.
The system and method given here allow provide for an exceptionally low-cost ( less than $500) dual-frequency receiver capable of centimeter-level performance in real time.